Below, there is a reminder of the principles governing the so-called AAL2 transmission protocol, described in the three ITU recommendations: I363.2, I366.1 and I366.2. This transmission protocol was defined for the purpose of avoiding the time problem involved in the assembly time of an ATM cell which is becoming critical for low rates. In fact, at 16 kbit/s, the assembly time amounts to 24 ms for the complete filling of the ATM cell. The solution selected was to multiplex the stream of several communications in a single ATM cell, using a packet information structure, called minicells or also CPS packets. This transmission method makes up the lower part of the protocol called the CPS (Common Part Sublayer). The indispensable adaptation functions are located above the CPS in the sublayers called SSCS (Service Specific Convergence Sublayer). The first, the SSCS segmentation sublayer is described in the ITU recommendation I.366.1 and is intended for the transmission of data units containing a significant number of bytes. The second, the SSCS trunking sublayer for real time is described in the ITU recommendation I.366.2.
A sequence of AAL2 packets is guaranteed on each AAL2 channel, but the service supplied by the CPS sublayer is a non-assured type, meaning that the missing packets (for example as a result of the loss of ATM cells transmitting them) are not replaced by retransmission at this level.
In FIG. 1a, there is a representation of the AAL2 packets of the CPS layer of the AAL2 protocol, as specified in the ITU 1.363.2 recommendation. The AAL2 packets have a 3-byte H_CPS header and a variable length P_CPS useable portion containing user information. By default, this length is limited to 45 bytes. As can be seen in FIG. 1a, the H_CPS header is made up in the following manner:
a CID connection identification field which is an 8-bit field and which allows the AAL2 connection to be identified,
a field, of LI length which is a 6-bit field and which codes the length of the useable portion of the packet in such a way that LI+1 is equal to the number of bytes,
a 5-bit user-to-user information (UUI) field,
an HEC field which is a 5-bit field for protection against header errors.
The AAL2 packets are generally transmitted by means of ATM cells.
There is a description below of the considerations with respect to the transmission of ATM cells on the E1/T1 type framed media. They are the subject of the ITU Recommendation G.804 entitled: “ATM Cell Mapping into Plesichronous Digital Hierarchy” in the case of complete use of the offered band, that is to say, the frame's 30 time slots—TI. Currently, they are implanted in specific ATM switching modules.
A time-division circuit frame such as the one addressed by recommendation G.704 is divided into 32 time slots each occupied by a byte. In this context, it is deemed that only slots 1 to 15 and 17 to 31 transmit user data. A frame has a length of 125 .mu.s, and the flow of data transmitted by each TI slot is limited to 64 kb/s. By regrouping the TI time slots, it is possible to allow the information to flow at N times 64 kb/s, i.e., 2,048 kb/s if one takes into consideration the 32 TI time slots, but 1,920 kb/s if one takes into consideration the 30 time slots used for the transmission of user data.
Each byte of an ATM cell is within a single time slot TI. There is no relation between the beginning of a frame and the beginning of an ATM cell. This is because the number of bytes in an ATM cell is different from that of a circuit frame.
FIG. 2 represents an example of ATM cell multiplexing on a frame structure such as the one which has just been described. The various frames are represented in stacks and numbered “frame n”, “frame n+1”, etc. They follow each other in this time-division order. Each time slot is referenced by its rank and by the number of eight-bit bytes used for the transmission of the corresponding data. This representation only has 30 TI time slots which are used (the number of eight-bit bytes thus vary between 1 and 30) and thus, not the time slots of the 0 to 16 ranking which are used for functions other than data transmission.
It is pointed out here that the ATM cells are packets of a length fixed at 53 bytes, of which 5 are assigned to the header. In FIG. 2, the eight-bit bytes of the ATM cells which are not empty are represented by a series of squares, each corresponding to a time slot in a frame. The headers of these cells are grayed out. With respect to the empty cells, they are not represented as squares (they are not partitioned).
One may note that the cells are consecutive inside each frame.
In order to allow for the transmission of both packet type and circuit type traffics on the same media, allowance was made for the use of only a single portion of the TI time slots for the transmission of the user data. This use was described in a specifications document produced by the ATM Forum and entitled “ATM on Fractional E1/T1: AF-PHY-0130.00” dated August 1999, and it consists of assigning, for the transmission of ATM cells, N slots at 64 kb/s among the frame's 30 possible time slots IT; the IT time slots which are not used for this purpose can then, in a traditional mariner, be used for the transmission of data in circuit mode, for example, telephone communications.
In FIG. 3, only a fraction of the time slots is allocated to ATM transmissions, these time slots TI being ranked in each frame between, on the one hand, 3 and 16, and 21 and 24 on the other. The time slots TI which are unused are represented without partitions. The number of eight-bit bytes used is included here between 1 and 18.
It is perfectly possible to transmit AAL2 packets, as described above, carried by ATM cells, themselves carried in time-division frames, such as those which are also described above. A frame of the E1/T1 type is, in this case, transparent to the cells' content. At the receiving end of the E1 connection, the ATM cells are first extracted for the time-division frame, then the AAL2 packets are extracted for the ATM cells to be reassembled. In a symmetrical manner, at the transmission extremity, the AAL2 packets are inserted into the ATM cells, which are then inserted into the time-division frame and thus transmitted.
The drawback to this solution is that the relationship between the number of useable bytes and the number of bytes transmitted over the connection is not very favorable in terms of efficiency. In fact, the 5 eight-bit bytes of the ATM cell headers added to a very variable number of padding eight-bit bytes associated with the multiplexing of mini-cells are so many time slots IT that they may be considered as lost with respect to the user data.
WO-A-00/59261 and EP-A-874 530 offer solutions to this specific problem.
In fact, the first involves a process for the transmission of mini-cells in a channel, E1 or T1 type, for example, made up of a number of multiframes, each multiframe containing, itself, several time slots. This process is set up in such a way that the transmission of these mini-cells does not pass through the ATM layer. In other terms, the heading of these ATM cells is suppressed and the mini-cells are directly transmitted to the E1 or T1 type frames. Nevertheless, this process necessitates the insertion of a starting eight-bit byte in the first time slot of each multiframe, the above-mentioned starting eight-bit byte containing a sequence number field, used to contain the number of multiframes.
Insofar as the EP-A-874 530 document is concerned, it sets forth the transmission of AAL2 type mini-cells to T1 type frames. To do so, the ATM layer is eliminated. There also, a six-bit pointer is used to define the starting location of the frame's next packet.